heat and a/c only blowing out of window vent
Sounds like one of your blend doors behind the dash is malfunctioning. You will probably want to take it to the dealer. Not a fun job.
The default for a failed HVAC system is Defrost & heat. There's the possibility that the vacuum line from the intake or from the reservoir is disconnected or broken, the check valve has failed or the control head is inop.
Search Mercury Grand Marquis Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Mercury Grand Marquis Experts